The invention relates to a method according to the preamble of claim 1 for elimination of radiation error in atmospheric temperature measurements.
The invention also concerns a temperature sensor structure.
One of the major sources of error in the temperature measurement of the upper atmosphere (by means of, e.g., radiosondes, rocket sondes and dropsondes) is the so-called radiation error of the temperature sensor. This type of error is caused by the drop of air density at higher altitudes, whereby convection heat transfer between the temperature sensor and air becomes weaker, while the relative proportion of radiation heat transfer simultaneously increases. Resultingly, the sensor temperature usually deviates from that of the ambient air, whereby it may be higher or lower depending on the local radiation circumstances in the atmosphere.
Heat transfer between the sensor and the atmosphere is described by equation: EQU H(T.sub.s -T)-.sigma..epsilon.AT.sub.s.sup.4 +.epsilon.R+.gamma.S=0 (1),
where
T.sub.s =sensor temperature (K) PA1 T=air temperature (K) PA1 H=convection heat transfer coefficient (W/K) PA1 .sigma.=Stefan-Bolzmann constant PA1 .epsilon.=sensor surface emissivity PA1 A=sensor area (m.sup.2) PA1 R=radiant power (W) of long-wavelength (thermal) radiation incident on the sensor PA1 .gamma.=sensor surface absorption coefficient for short-wavelength (solar) radiation PA1 S=solar radiation power (W) incident on the sensor. PA1 1) Only the ratio of solar radiation absorption coefficients of the coating systems used need be known. This value is essentially easier to measure than the absolute values of absorption coefficients and emissivities of three different coating systems. Furthermore, the ratio of absorption coefficients can be measured from ready-made, ready-coated sensors, while the absolute values of absorption coefficients and emissivities can be measured from planar samples only. Accordingly, the present method provides a much more accurate result. PA1 2) The equipment for absorption coefficient ratio measurement is relatively simple and quick to use, whereby the ratio measurement can be made for each manufactured sensor pair as a step of the production process. This facility adds further to the accuracy of the sensor pair. PA1 3) The thermal heat transfer coefficient H is not needed in computation, because it will be eliminated from the equation pair. Accordingly, any possible inaccuracy related to the value of this variable will not affect the end result. PA1 4) The number of sensors required in the sensor system is reduced by one.
The first term -H(T.sub.s -T) of the equation represents convection heat transfer. The last three terms represent radiation heat transfer. The term -.sigma..epsilon.AT.sub.s.sup.4 represents the thermal emission loss component of the sensor, while the term .epsilon.R represents the component of thermal radiation (that is, long-wavelength radiation, .lambda..apprxeq.3-40 .mu.m) absorbed by the sensor. The term .gamma.S represents the component of solar radiation (that is, short-wavelength radiation, .lambda..apprxeq.0.2-3 .mu.m) absorbed by the sensor. The component of convection heat transfer has been assumed negligible.
Radiation error can be reduced by making the dimensions of the sensor the smallest possible, whereby the ratio of convection heat transfer to radiation heat transfer is accentuated. Another approach is to coat the sensor with a coating system of the smallest possible absorption coefficient. Both of these methods are used in conventional sensor embodiments. However, the radiation error cannot be entirely eliminated by these means, because the dimensions and absorption coefficient of the sensor cannot be made indefinitely small.
A method of radiation error elimination different from those described above is based on using three sensors of identical structure and dimension but coated with different coating systems. Each of the coating systems has a different emissivity and coefficient of absorption for solar radiation. Correspondingly, the parallel sensors have different radiation errors and indicate different temperatures, whose values are dependent on the radiation circumstances of the atmosphere. Then, a separate heat transfer equation (1) can be written for each sensor, whereby a set of three equations results with four unknown variables: T, R, S and H. Of these, however, the convection heat transfer coefficient H can be solved with relatively good accuracy if the shape and dimensions of the sensor are known. The rest of the unknown variables, the actual temperature T of the atmosphere inclusive, can be solved from the set of equations.